Depending on the age of the kitten, it's perfectly normal for the eye colour to beging to change from the kitten-blue to the colour the eyes will be when they are adults. This usually occurs anywhere from 5 weeks to 4-5 months.
However, even an adult cat's eyes can sometimes have variations in colour. My cat, Gerona, for example will sometimes have predominately green eyes rather than her usual yellow. It depends on her mood and the lighting she's in. So, I would say slight changes in eye colour is nothing to be worried about.